1. Field of Invention
The invention relates to a control apparatus for an automatic transmission of a vehicle, and more particularly to a neutral control for decreasing an engagement pressure of a neutral control clutch of the automatic transmission, which performs connection and disconnection between the automatic transmission and an engine of the vehicle when the vehicle stops in a state where the forward running range is selected.
2. Description of Related Art
It has been well known that a control apparatus for controlling an automatic transmission in a vehicle reduces an engagement pressure of the clutch for neutral control of the automatic transmission, which clutch performs connection and disconnection between the automatic transmission and the engine, that is, a neutral control when the vehicle stops in the forward running range, for the purpose of improving the fuel efficiency (disclosed in the publication, for example, JP-A-10-196782).
As the engagement pressure of the neutral control clutch is decreased, the road wheels are brought into a freely rotatable state. Accordingly the control apparatus disclosed in the aforementioned publication JP-A-10-196782 engages a hill-holding engagement element that inhibits counter rotation of an output shaft of the automatic transmission so as to prevent the vehicle from moving backward on the up-hill, and executes the neutral control for reducing the engagement pressure of the neutral control clutch.
When the automatic transmission is returned to the state where the forward drive is established upon completion of the neutral control, the engagement pressure of the neutral control clutch is returned to the original state, and the hill-holding engagement element is disengaged. In this case, an appropriate correlation between the timing at which the neutral control clutch is engaged and the timing at which the hill-holding engagement element is disengaged has to be maintained. If the engagement of the neutral control clutch is completed at relatively earlier stage with respect to the timing at which the hill-holding engagement element is disengaged, re-engagement of the neutral control clutch may cause a certain shock. Meanwhile, if the hill-holding engagement element is disengaged at relatively earlier stage with respect to the timing at which the neutral control clutch is engaged, the vehicle stopped on the up-hill may be moved backward momentarily.
When there is a variation in the friction coefficients of individual neutral control clutches, or a change in the friction coefficient owing to passage of time, the time taken for engaging the neutral control clutch may vary even if the engagement pressure is controlled in the same way as described above. In the control apparatus disclosed in the publication JP-A-10-196782, a gradient of increase in the engagement pressure is set by learning such that the neutral control clutch can be engaged for a predetermined time irrespective of the variation of the friction coefficients in individual neutral control clutches, change in the friction coefficient owing to passage of time, and the like.
The publications, each of which is considered as being relevant to the invention, will be listed as below:    (1) JP-A-10-196782,    (2) JP-A-2003-156142, and    (3) JP-A-2003-156143.
Even if each of the neutral control clutches is controlled to be engaged for a predetermined time, the neutral control clutch is engaged at relatively earlier stage or the hill-holding engagement element is disengaged at relatively earlier stage, depending on the time for implementing the neutral control. As a result, when the automatic transmission returns from the neutral state to the one where the forward drive is established, a certain shock may occur. This may also move the vehicle stopped on the up-hill backward momentarily.